1. Field of the Invention
Example embodiments relate generally to identification resistors in a wire harness for cordless power tools.
2. Description of Related Art
Cordless products which use rechargeable batteries are prevalent throughout the workplace as well as in the home. From house wares to power tools, rechargeable batteries are used in numerous devices. Ordinarily, nickel—cadmium (NiCd), nickel metal hydride (NiMH) and/or Lithium-ion (Li-ion) battery cells are used in these devices.
Various battery technologies can be damaged when discharged in excess of the manufacturer's recommendations. Accordingly, circuitry to prevent current flow is required to protect the pack when a battery voltage drops below a given voltage threshold, referred to as an under-voltage lockout. For example, a protection circuit in the battery pack and/or tool can sense the battery voltage, and if the voltage drops below a given voltage level the circuit directs turning off of , a discharge semiconductor device (e.g., a discharge FET) in the pack. As a result, battery cells are still susceptible to charge but will not discharge.
Accordingly, conventional battery unit with charge/discharge control and over-discharge protection is designed primarily for low-voltage portable electronic devices. Such devices are characterized by using battery packs of secondary batteries cells (such as Li-ion, NiCd, NiMH) that provide a maximum output voltage of about 4.2 volts/cell, for example.
However, much higher voltages than described above are required for power electronic devices such as cordless power tools. Accordingly, modified NiCd battery packs that provide the same or greater power at lower weight, and Li-ion battery packs which may provide higher voltage outputs than current Li-ion batteries, and at a much reduced weight (as compared to NiCd or NiMH), are being developed. A characteristic of these battery packs is that both batteries may exhibit substantially lower impedance characteristics than conventional Li-ion, NiCd and NiMH batteries.
However, as these battery technologies advance, the introduction of lower impedance chemistries and construction styles to develop secondary batteries generating substantially higher output voltages (such as at least 11 V and up, for example) can create compatibility issues with existing cordless power tools. Battery packs having lower impedance also means that the pack can supply substantially higher current to an attached electronic component, such as a power tool. As current through a motor of the attached tool increases, demagnetization forces (e.g., the number of armature turns of the motor times the current, ampere-turns) could substantially increase beyond a desired or design limit in the motor. Such undesirable demagnetization could thus potentially burn up the motor.
For example, a lower impedance electrical source could cause damage to a tool's motor when the tool is held at stall condition. During motor stall, the motor and battery impedances are the only mechanisms to limit the current since there is no back-EMF created by the motor. With a lower impedance pack, the currents would be higher. Higher currents through the motor might cause a stronger de-magnetization force than what the tool's permanent magnets were designed to withstand.
Additionally, start-up of the tool could produce excessive starting currents and cause demagnetization of the motor. Thermal overload could also be a result of using a low impedance electrical source in an existing power tool, as the new batteries may be designed to run longer and harder than what the original cordless tool system was designed. Accordingly, over-discharge or current limiting controls may need to be in place before these developing lower-impedance batteries may be use with existing cordless power tools, for example.
One approach has been to use identification (ID) resistors in the tools to identify the information received from the battery pack. The battery pack may be configured for communicating and sensing information from the tool through communication terminals in the pack. Upon communication of this data, the ID resistors act to place restrictions of the maximum power and current through the battery pack. However, most conventional ID resistors are large and take up a substantial amount of space in the power tool.